1. Field of the Invention
The present invention relates to broadband antennas. More specifically, the present invention is directed to antennas that are small compared to the operating wavelength over much of the frequency band of operation. The invention further relates to a means of reducing the size of a conical radiating resonator in a manner so that a collection of such resonators provides a repetitive variation in input impedance. The amount of the variation in impedance can be controlled by the selection of lumped tuning elements. The invention provides a means of switching the tuning elements in a manner that yields several wide operating bands having similar performance characteristics, thereby providing an electrically small antenna that can operate across a very wide range of frequencies.
2. Description of the Related Art
For a number of years now radio communication systems have been increasing in complexity and numerous different communications services may be employed by a typical user, even a typical member of the general public. Furthermore, an increasing variety of communications tools is available and in use by the average consumer. Therefore, individuals are using a greater number and wider range of frequencies for these communication purposes. For example, a typical person in day-to-day tasks may use AM and FM radios, cellular telephones and, more recently, GPS systems. This ever-increasing trend in the use of communication devices is not likely to change.
The explosion in the use of communications technology is having an impact on the antennas that are an integral part of the every radio system. However, there are currently no known single, small antenna systems available that can operate as a practical matter across the varied range of frequencies that are currently in use by individuals on a regular basis.
Multiple services may operate on widely disparate frequency assignments. Some systems use spread-spectrum or frequency agile techniques that need much wider instantaneous bandwidths than those used with older modulation methods. The examples set forth above cover the kilohertz range through low gigahertz frequencies. Moreover, this push for wider bandwidth is accompanied by a desire to reduce the physical size of the antenna commensurate with the reductions that have been achieved in the size of the electronic components of the systems that use them. Currently, each of the systems mentioned above typically employs a separate dedicated antenna. As radio communication systems become more integrated, particularly those in vehicular services, it is desirable to employ a single antenna for all functions of the system. However, none are currently available to provide the necessary range of operating capability.
A review of known small-antenna designs confirms this fact. A comprehensive account of the state-of-the-art in small antenna design at that time was given in Proceedings of the ECOM-ARO Workshop on Electrically Small Antennas, G. Goubau and F. Schwering (eds.), Fort Monmouth, 1976. The small antenna art in more recent years is summarized in Small Antennas, K. Fujimoto, A. Henderson, K. Hirasawa and J. R. James, Wiley, New York, 1987. Two principal methods of reducing antenna size, reactive loading and material coating, are discussed. Since loading with reactive elements reduces the bandwidth of the antenna, resistive loading is often used to regain the lost bandwidth. However, resistive loading results in loss of efficiency and gain.
A Study of Whip Antennas for Use in Broadband HF Communication Systems, B. Halpern and R. Mittra, Tech. Rep. 86-1, Electromagnetic Communication Laboratory, University of Illinois, Urbana, 1986 gives an example of one of many attempts that have been made to use lumped loading elements to substantially reduce the length of a whip antenna while retaining the ability to cover a wide range of frequencies. Not only is it difficult to maintain coverage of wide bandwidths with whip antennas, but the problem is compounded by using loading elements to shorten them. Hence, this approach has not been very successful when an objective of the design has been to produce a structure with low profile, a feature that is particularly desirable for vehicular antennas.
A new approach to low-profile antennas that are electrically small was introduced in Series-Fed, Nested, Edge-Loaded, Wide-Angle Conical Monopoles, P. E. Mayes and M. O""Malley, Digest of IEEE Antennas and Propagation Society International Symposium, Ann Arbor, Mich., 1993. It was shown there that a conducting cone with apex angle near ninety degrees, even though quite small in terms of the wavelength, could, at a certain frequency, display zero reactance (resonance) at the input terminals. The cone was fed against a ground surface from a coaxial cable (center conductor to tip of cone, shield to ground). The reduction in size was achieved by placing lumped inductive loads between the rim of the cone and the ground surface. It was also shown there that two such cones could be nested, connected in series, fed against ground to a transformer in such a way that low values of reactance could be maintained over a band of frequency. Additional data on edge-loaded conical monopoles are given in Experimental Studies of Two Low-Profile, Broadband Antennas, M. F. O""Malley and P. E. Mayes, Electromagnetics Laboratory Report 94-6, University of Illinois, Urbana, 1994.
A resonant radiator formed by the space between two nested open-ended conducting cones is one basic prior-art element that is used in the present invention. A single radiator of this form is shown generally in cross section at 10 in FIG. 1A wherein the polar angle defining cone 11 is ninety degrees. This is an example of the special case where the member 11 is actually a planar circular disc. Accordingly, as used in this specification, the term cone can mean either a metal plate or an open-ended angled cone. The second or upper cone 12 of smaller polar angle is positioned above the lower member 11 with cone 12 having a tip 18 at the center of cone 11 and with the axis of cone 12 substantially coincident with the normal through the center of cone 11. A small circular aperture 14 is provided in cone 11 with its center substantially coincident with the center of cone 11. A coaxial cable 15 is attached to the antenna so that the shield 16 of the cable 15 is electrically connected to the rim of the aperture 14. The center conductor 17 of the coaxial cable 15 is electrically connected to the tip 18 of cone 12. Alternatively, this connection may be accomplished with a panel jack having a center PIN connected to tip 18 of cone 12. The outer conducting shield of the panel jack may be attached to the rim of the aperture 14.
Networks of one or more lumped elements 20 are positioned at respective locations 21a, 21b, 21c, 21d spaced around the periphery of the conical antenna between the upper cone 12 and the lower cone 11 as shown in FIG. 1B. The networks are electrically connected to the upper and lower cone members 11 and 12 as shown in FIG. 1A. Usually, several similar networks will be distributed around the periphery of cone 12 in order to render sufficient symmetry to the system to maintain in azimuth the desired degree of uniformity in radiation.
Continuous electronic tuning of an edge-loaded conical resonator was demonstrated in Tunable, Wide-Angle Conical Monopole Antennas with Selectable Bandwidth, P. E. Mayes and W. Gee, Proceedings of the Antenna Applications Symposium, Allerton Park, Ill., 1995. The frequency of the high-impedance resonance was varied by placing voltage-variable capacitors (varactors) in series with the inductors on the rim of the cone. FIGS. 2A-2C show possible design choices for the network elements of the prior art. FIG. 2A shows a network comprised of a single inductor 32 as taught by O""Malley and Mayes. FIG. 2B shows an inductor 33 in series with a varactor 34 as used by Mayes and Gee. For a given bias voltage, the network of FIG. 2B is equivalent to the inductor 35 in series with a capacitor 36 as shown in FIG. 2C.
FIG. 2D is an approximate equivalent circuit for the conical radiating resonator of FIG. 1. Since the wave launched between any two coaxial cones is transverse electromagnetic (TEM), the region between the tip and rim of the cone can be represented by a section of uniform transmission line 41 having length 55 equal to the tip-to-rim distance. The line is terminated by the lumped element 20 that represents the net reactance at the rim of the cone and by a resistor 42 that simulates the radiation from the space between the two cones.
The experimental results shown in FIG. 3 indicate that a particular conical radiating resonator of the type shown in FIG. 1 could be tuned from 120 to 260 MHz by changing the varactor bias voltage from zero to 23 volts. For some applications, however, this tuning range (2.17:1) is far from adequate. This is especially true if the antenna is required to provide coverage for a plurality of the services mentioned above.
Furthermore, it was later noted that the combination of inductor and varactor in series produced a rim load with a reactance that varied much more rapidly with frequency than that of the inductor alone. Although it would be theoretically possible to achieve a wide instantaneous bandwidth by using multiple resonators with overlapping bands, more resonators would be required when inductor-varactor loading is used than when the loading is only inductive. In addition, the varactor-tuned system could not be tuned with adequate accuracy in face of time and temperature variations. This follows from the need for the resonant frequencies of the several resonators to be related to one another in a way that preserves the shape of the bandpass characteristic.
Devices of the prior art have been shown to have substantial shortcomings particularly if they are to be used with a plurality of services that employ a wide range of transmission frequencies. In order to provide a single antenna structure that is capable of servicing a wide range of frequencies, it is desirable that the structure be capable of electrical tuning across the different ranges of frequencies to be serviced by the device. Hence, there is need for a simple means of adjusting the coverage in such a manner that a single antenna system can be used over a wider range of frequencies than in the past.
Thus, there remains a need in the art for an antenna that is physically small, has a wide instantaneous bandwidth, and which can be electrically tuned over a still wider range of frequencies. It is therefore an object of the present invention to provide a means of realizing an electrically small antenna with a minimal number of resonant radiators that has several wide instantaneous bands that can accessed quickly and accurately. Additionally, it is a further object of the present invention to provide an electrically small antenna that may be switched to enable a single antenna to operate over a very wide range of frequencies. Other objects and advantages of the present invention will be apparent from the following summary and detailed description of the preferred embodiments.
The antenna structures of the present invention produce wider instantaneous bandwidth with a given number of conical radiators than is possible using varactors in series with lumped inductance edge loads as disclosed in the prior art. In one aspect of the design, several wide instantaneous bands are available from the same antenna system and they can be accessed quickly and accurately simply by electrical switching. By placing the switched bands adjacent to one another, the antenna system of this invention can cover an extremely wide range of frequency. Advantageously, the switched bands can be chosen to coincide with the separate bands of certain communication services.
The present invention employs a resonant radiator of conical shape with an input impedance that has a large resistive value at a predetermined frequency (resonance) where the maximum dimension of the resonator is small compared to the operating wavelength. The reduction in size is obtained by placing one or more reactive elements at the outer extremity of the radiator. Several radiators are connected in such a manner (series) that the impedance observed at the input port of the system is the sum of the impedances of the individual radiators. The resonances of the individual radiators are chosen to adjust the antenna performance according to desired specifications. For example, the resonances can be made close to one another so that the variation with frequency of the input impedance is minimized. The instantaneous bandwidth of an antenna system that maintains the same level of impedance variation will depend upon the number of resonators in the system.
It is important, therefore, when wide instantaneous bandwidths or very small impedance excursions are desired, to use the reactive loads that provide the needed versatility with a minimum change in reactance with frequency. It has been discovered that switching fixed elements is superior to continuously tuned ones in this regard. Not only is the bandwidth of each resonator adversely affected by the rapid variation of the reactance of series LC tuning elements, but the integrity of the performance versus frequency depends upon the ability to maintain an exact relationship among multiple resonators that are needed to provide a wide instantaneous bandwidth.
In accordance with the present invention, a plurality of open-ended conical radiating resonators employs inductors or capacitors in series with PIN diodes. Application of a variable dc voltage across the PIN diodes allows the antenna structure to be tuned over a very wide band of frequencies.
Another advantage of the present invention is the ability to quickly switch the antenna from coverage of a certain band to coverage of another non-adjacent band. Discontinuous tuning by means of varactors requires the application of a discontinuous bias voltage. Generating such a bias voltage would be an added complication in the system. The antennas of the present invention can be designed so that the switched bands coincide with the desired bands. This remains true even when the desired bands are beyond the range of a varactor-tuned system.